CONTROLS ON MINERALIZATION OF REES AND OTHER CRITICAL MINERALS WITHIN THE PERMIAN PHOSPHORIA FORMATION, SOUTHWEST MONTANA

The Phosphoria Formation, deposited in the early Permian ~270 Ma, extends throughout the Western Rocky Mountain region, including Montana, Idaho, Wyoming and Utah. The formation comprises multiple sequences of chert, bitumen-rich shales, and phosphorite. The Phosphoria has been mined historically since the 1910s for phosphate used in fertilizer and chemical production. These mines spanned across SW Montana and through NE Utah, mostly mining the upper Retort member in Montana and the Meade Peak member in Idaho and Utah. REE mineralization in the Phosphoria has been noted historically, but with recent economic and political pressures for domestic REE production, a deeper understanding of the location and quantities of REE mineralization has become a priority. This study examines the controls of mineralization of critical minerals within the Phosphoria (including REEs, V, and Cr) to use as a vector for more focused exploration of phosphorite-hosted orebodies. Stratigraphic sections of Phosphoria outcrops around Montana have been measured and sampled to examine diagenetic history in thin section and to compare those data to whole rock geochemistry assays, and to mineralogical data collected by automated scanning electron microscopy (SEM). Mine waste piles from legacy mining activity of the Phosphoria have also been evaluated for REE mineralization in the same fashion. Assays reveal total REE + Y grades up to 3480 ppm (~0.35 wt.%) with most samples averaging ~1000 to 2000 ppm. Heavy REEs represent 40 – 50 % of the total REE grade in the phosphorite. Interestingly, while the REEs are located in the phosphate minerals (mostly francolite), there is a variable correlation between phosphate and REE grades. Vanadium (V) grades from Paris Hills, Idaho range up to 7510 ppm. These values are elevated compared to phosphorite samples throughout Montana where V grades rarely exceeded 1000 ppm.